1. Field of the Invention:
The invention relates to an advantageous process for the liquid-phase preparation of nitriles from aliphatic dicarboxylic acids, in which, apart from phosphoric acid or polyphosphoric acid as the catalyst, an adsorbent is additionally used.
2. Discussion of the Background
Aliphatic dinitriles have gained considerable economic importance as starting materials for the preparation of the corresponding diamines. They are easily accessible from aliphatic dicarboxylic acids by reaction with ammonia at elevated temperatures in the presence of a dehydration catalyst. The preparation of nitriles from heat-stable acids is preferably carried out in the gas phase over a fixed-bed catalyst at temperatures between 300.degree. and 550.degree. C. In the reaction of longer-chain acids, in which decomposition reactions take place increasingly with increasing temperature, the liquid-phase preparation of nitriles is used. In this process, the reaction is carried out between 200.degree. C. and 350.degree. C., at which temperature the acid is present as a melt, and ammonia is introduced into the melt in the presence of a catalyst (D. T. Mowry, "The Preparation of Nitriles", Chem. Rev. 42 (1948), 191-285). In the liquid-phase preparation of nitriles, catalysts based on phosphoric acid are preferred. According to German Patent No. 734,558, phosphoric acid, or partially or completely neutralized phosphoric esters, are used for preparing adiponitrile, which is obtained from adipic acid in a maximum yield of 86.6% of theory. The catalysts used in the process have considerable disadvantages. Phosphoric acid is converted by elimination of water to polyphosphoric acid, which is deposited in the reactor. The polyphosphoric acid impairs heat transfer from the external source to the reaction mixture, and furthermore, causes corrosion. The phosphoric esters also mentioned in German Patent No. 734,558 result in the same problems, since they are hydrolysed under the conditions of nitrile preparation to form phosphoric acid, which in turn, forms polyphosphoric acid.
According to German Patent No. 857,194, a process for the preparation of dinitriles from aliphatic .alpha.,.OMEGA.-dicarboxylic acids in the presence of phosphoric acid or salts thereof is described. In the presence of metaphosphoric acid, sebacic acid is converted to sebaconitrile in a yield of 82.1% of theory. The catalysts phosphoric acid, metaphosphoric acid or salts thereof used in this process also lead to the formation of polyphosphoric acid, resulting in the above-mentioned difficulties with regard to deposits forming on the walls of the reactor.
U.S. Pat. No. 3,297,740, which is incorporated herein by reference, provides a further example of the many nitrile preparation processes from dicarboxylic acids in the presence of phosphoric acid. Crude trimethyladipic acid is reacted with ammonia in a stirred reactor in the presence of phosphoric acid or phosphoric acid salts at temperatures between 240.degree. and 290.degree. C. to give trimethyladiponitrile, while simultaneously, the reaction side-products are removed by distillation. The top side-products formed are ammonia-containing water and more volatile byproducts such as, in particular, trimethylcyclopentanone. Trimethyladiponitrile is discharged via a side outlet.
Crude trimethyladiponitrile also contains trimethyladipimide. During the subsequent washing with ammonia, it is converted into the corresponding diamide, which is passed to the reactor, and converted quantitatively into trimethyladiponitrile. Before vacuum distillation of trimethyladiponitrile, the imides of the shorter-chain acids originating from the crude trimethyladipic acid have to be removed by washing with sodium hydroxide solution. In this nitrile preparation process too, deposits of polyphosphoric acid are formed, which cause shutdowns and/or breakdowns in the reactor, and subsequent loss of production, as a result of the above-mentioned deposits of polyphosphoric acid on the walls of the reactors.
Surprisingly, it has now been found that deposits of polyphosphoric acid on the reactor walls formed in the liquid-phase preparation of nitriles from dicarboxylic acids in the presence of phosphoric acid, polyphosphoric acid and metaphosphoric acid can be prevented by adding an adsorbent to the reaction mixture.